This disclosure is related generally to the field of marine surveying. Marine surveying can include, for example, seismic and/or electromagnetic surveying, among others. For example, this disclosure may have applications in marine surveying in which one or more geophysical sources are used to generate energy (e.g., wavefields, pulses, signals), and geophysical sensors—either towed or ocean bottom—receive energy generated by the sources and possibly affected by interaction with subsurface formations. Geophysical sensors may be towed on cables referred to as streamers. Some marine surveys locate geophysical sensors on ocean bottom cables or nodes in addition to, or instead of, streamers. The geophysical sensors thereby collect survey data which can be useful in the discovery and/or extraction of hydrocarbons from subsurface formations.
Historically, geophysical sources have been towed at or near the water surface. Flotation and towing systems have positioned the sources vertically (i.e., water depth), laterally (i.e., in the cross-line direction; horizontal and perpendicular to the local towing direction), and axially (i.e., in the in-line direction; horizontal and parallel to the local towing direction). Near-surface towing has provided easy access for power and data transfer. Also air gun signals (e.g., bubbles) may be attenuated by higher water pressures at greater depths. However, deep towing of sources may provide better data quality in many instances. For example, deep towing of sources may reduce source ghosting concerns. As is commonly known, acoustic energy that is reflected downwardly from the water surface is commonly known as a “ghost” signal. This causes a sequence of notches in the acquired data spectrum, including a notch at zero frequency (0 Hz). The frequencies of these notches in the detected acoustic signal are related to the depth at which the geophysical source is towed. Low-frequency data acquisition may be improved by towing the sources at a greater depth. However, this causes the ghost notches in the spectrum to occur at lower frequencies, and hence limits the high frequency parts of the spectrum needed for high resolution imaging of shallower targets. Also, when using air gun(s) as a seismic energy source, the fundamental frequency of the gun(s) increases with increasing depth. Hence, the increase in energy in the low frequency end when towing the air-guns deeper due to the source ghost, is counteracted by the increase in fundamental frequency of the air-gun(s).
In some instances, surface obstructions impede shallow towing of sources. At times, data may be desired from sources at more than one depth and/or horizontal position. Towing systems may sometimes suffer from vibration or other sources of noise. It would be beneficial to steer sources and/or source arrays in conjunction with and/or independently of existing flotation and towing systems.